Sacrifice your health to heal your allies then regen it back!

Your general aim in the game, although it sounds obvious is to use your Astral Infusion(W) on allies to heal them (preferably just after landing a Starcall(Q) so you can give them the regen and movement speed buff). The heals will cost you 10% of your max health, so beware, 10 heals and your dead (and with a 1.2 second cooldown in the late game, that's not hard to do). What you want to build is a bunch of health regen items to keep that missing health up as well as some nice goodies for your team like Locket and Ohmwrecker.

Use your Wish(R) to tip the scales of a teamfight, help team mates who are just out of reach or perhaps even on the other side of the map (when that top laner goes for that deep tower dive).

Use your Starcall(Q) and Equinox (E) to zone enemies when in lane or silence groups of them in team fights.

Quintessences
3x Greater Quintessences of Gold
I take these to increase my gold income. Although I will be getting the ancient coin, as well as taking bandit, the extra gold from these quints is quite handy, especially considering you will not be getting any CS or Kills, most of your cold should be coming through, runes, masteries and assists.
9x Greater Mark of Scaling Health
These are what will increase your health and make the 3000 HP mark that little bit easier to achieve for your Warmogs.
9x Greater Seal of Scaling Health Regeneration
This is what will help keep your health up when you sacrifice it to heal others. A large portion of this build and guide has a focus on health regeneration, and this just adds to that.
9x Greater Glyph of Ability Power
A little something to boost those heals. It's not a lot but I find it helps. (you could conciser switching these out for MR runes.

These masteries boost your healing output as well as, most importantly, increase your Health regeneration. For the most part, these are pretty standard for Soraka. There isn't much worth mentioning, however you may want to consider switching bandit for Dangerous game, if you are not matched up against a melee support or find it hard to basic attack enemies often.

Generally speaking, you want to get one point in each ability first, Q>E>W. Then you want to max your Starcall(Q) pre-ult, so then Q>Q>R. From then on, put every point you can into Astral Infusion(W) leveling up Starcall(Q) when you don't have a choice and then finally finish off your Equinox(E).
You have 2 options at level 2, you can choose Astral Infusion(W) or Equinox(E). Most of the time, you won't need heals that early in lane and are able to get more utility via the zoning of Equinox(E), however if your ADC is taking some early damage then get your Astral Infusion(W) second and max it after Wish(R).